The field of electrical connection and termination has, until recently, largely depended upon traditional methods such as soldering and crimping to effect the connection and termination of, for example, conductors and cable screens. In simple applications both of these traditional methods are quite satisfactory, but there are many other applications, for example, when the connection or termination must be made in a cramped environment or when one or more associated components are fragile or vulnerable to heat, when a highly skilled operator is needed to obtain successful results.
More recently, it has been proposed to utilize the properties of so-called "memory metals" (sometimes called heat-recoverable metals) to form connections. Such memory metals and their properties are described, for example, in British Pat. Nos. 1,202,404; 1,327,441; 1,327,442 and 1,338,278 and in U.S. Pat. Nos. 3,174,851 and 3,351,463. Basically, these memory metals are alloys which are capable, like certain plastics materials, of having the property of heat recoverability imparted to them by virtue of their different properties in their martensitic (low temperature) and austenitic (high temperature) states. Thus an article made from such a memory metal can easily be deformed while in the martensitic state to a heat-unstable configuration in which it will remain while kept in the martensitic state. When it is warmed through the transition temperature, (which, in practice, is usually a small temperature range) to the austenitic state, however, it will recover towards its original form.
As disclosed in British Pat. No. 1,327,441, such memory metals can be used to form useful heat-shrinkable connecting members, especially tubular connecting members with inner teeth.
As disclosed in British Pat. No. 1,327,442, some of the recovery can be made reversible by imparting further secondary or non-thermally recoverable deformation to the article. Thus when a heat-shrunk memory metal connector is once again cooled to its martensitic state a small degree of re-expansion may occur. However, the degree of re-expansion is generally rather small (i.e. not greater than 3% compared to a recovery of about 8%) and, in some applications, is not sufficient on its own to facilitate removal and reuse of the connector.
In U.S. Pat. No. 3,740,839 there is described and claimed a reusable connector comprising a heat-recoverable metal member (e.g. a band) in conjunction with a resilient connecting member (e.g. a longitudinally slotted cylindrical element positioned inside the band). When the temperature is above the transition temperature the recovery force of the recoverable metal band dominates and a connection is made on to an underlying object. When, on the other hand, the band is cooled to its weaker martensitic state, the resilient forces of the tines of the connecting member are dominant and the band is forced to expand, thus releasing any connection which has been made. In an alternative form of connector the heat-recoverable metal member may be employed to force the tines of the resilient connecting member apart on recovery, thus enabling a different type of connection to be made (or broken) at the higher temperature. The connector described and claimed in U.S. Pat. No. 3,740,839 has wide application and has proved very successful in solving several different types of problems traditionally encountered in the general field of connection and termination but there are nevertheless some applications where its use is not suitable.
One example of a situation in which the use of a connector as described and claimed in U.S. Pat. No. 3,740,839 may not always be suitable is when the requirements of the connection or termination are such that no resilient connector member could be used because the resilience of the member could not be accommodated by other components associated with the connection. Another example is when the other requirements of the metal of the connector, such as coefficient of thermal expansion, thermo-electric properties and solderability, are not compatible with the necessary degree of resilience. Yet another disadvantage to the connector with a resilient member is that disengagement of the connector is more likely if the connector is temporarily exposed to a temperature at which the recoverable member reverts to its martensitic form.